Steel wire for steel cord used as reinforcement material for automobile radial tires, various belts, and hoses or steel wire for sawing wire is generally produced by hot rolling a steel billet, then controllably cooling it to obtain a steel rod (rolled rod) of a diameter of 4 to 6 mm, and drawing this rolled rod to a diameter 0.15 to 0.40 mm ultrafine wire. Further, these ultrafine steel wires are twisted together to form steel wire strands to thereby produce steel cord.
The drawing process comprises drawing the 4 to 6 mm rolled steel rod by primary drawing to a diameter of 3 to 4 mm, then intermediate patenting it and further drawing it by secondary drawing to a 1 to 2 mm diameter. After this, final patenting, brass plating and final wet drawing are performed. Final diameter of steel wire is 0.15 to 0.40 mm.
In recent years, to reduce production costs, intermediate patenting has been omitted and the rolled rod after controlled cooling has been drawn directly up to the final patenting wire diameter of 1 to 2 mm in increasing cases. Therefore, direct drawability from the rolled rod, is being demanded. The ductility and workability of the rolled rod are then becoming important.
The index showing the ductility of the steel rod, that is the area reduction, depends on the austenite grain size. It rises as the austenite grain size is refined. Attempts have been therefore made using Nb, Ti, B, and other carbides and nitrides as pinning particles so as to refine the austenite grain size.
For example, Japanese Patent Publication (A) No. 8-3639 discloses an art of including one or more of Nb: 0.01 to 0.1%, Zr: 0.05 to 0.1%, and Mo: 0.02 to 0.5% as additive elements so as to further increase the toughness and ductility of ultrafine steel wire.
Japanese Patent Publication (A) No. 2001-131697 also proposes refining the austenite grain size using NbC.
However, these additive elements are expensive, so cause cost increase. Further, Nb forms coarse carbides and nitrides and Ti forms coarse oxides, so there have been cases of breakage if drawing up to a thin wire size of a diameter of 0.40 mm or less. Further, according to verification by the inventors, it has been confirmed that with BN pinning, refining of austenite grain size to a degree having an effect on the area reduction rate is difficult.
On the other hand, as shown in Japanese Patent Publication (A) No. 8-3639, there is proposed an art of reducing the patenting temperature to control the structure of the steel rod to bainite and thereby increase the drawability of a high carbon steel rod. However, in order to make a rolled rod a bainite structure in-line, it is necessary to immerse it in molten salt. This treatment causes high costs and simultaneously is liable to reduce the mechanical descaling performance.